This invention relates to an image reader using photoelectric conversion elements for reading an original image and in particular to an image reader that can vary resolution.
An image reader is realized, for example, as an input part of a copier or a facsimile machine or as an image scanner connected to a computer or a network. The image reader has various read modes, one of which can be selected by the user. The available read modes include resolution selection, magnification setting, etc.
A method of changing resolution in the vertical scanning direction by changing the transport speed of an original and the number of thinned-out lines in combination is known, for example, as disclosed in the Unexamined Japanese Patent Application Publication No. Hei 4-369163, as a resolution changing method in a conventional image reader. Generally, a method of thinning out pixels is used to change resolution in the horizontal scanning direction. However, such techniques for changing resolution cannot meet recent demands for speeding up and high resolution of image readers; a technology satisfying such demands is in need of development.
FIG. 9 is an illustration to show an example of the relationship between resolution and setup magnification and vertical scanning speed and interpolation lines in a conventional image reader. In this example, the resolution can be selected from among three levels of 400 spi, 200 spi, and 100 spi and the magnification can be set in the range of 50% to 400%. The vertical scanning speed is the original transport speed or the photoelectric conversion element move speed.
For example, mode 2 means that the resolution is 400 spi and that the vertical scanning speed is changed in response to the setup magnification 50%-400%. That is, as the setup magnification is nearer to 50%, the vertical scanning speed is increased and the number of samplings is decreased, reducing an image. As the setup magnification is nearer to 400%, the vertical scanning speed is slowed down and the number of samplings is increased, enlarging an image.
If the resolution is set to a half of the resolution 400 spi, namely, to 200 spi, the vertical scanning speed is set to twice the speed responsive to the setup magnification and the number of samplings in the vertical scanning direction is halved for lowering the resolution, as shown in mode 3. However, there is also a limit of speeding up the vertical scanning speed; in the example, the limit is reached when the setup magnification 100% with the resolution 100 spi is selected. As shown in mode 5, if the setup magnification is smaller than 100% with 100 spi, instead of multiplying the vertical scanning speed by four, the vertical scanning speed is doubled and two lines are interpolated or one line is thinned out for decreasing the number of lines.
In the example, in mode 2 to mode 5, photoelectric conversion elements are moved for executing vertical scanning and in mode 6 to mode 11, an original is moved for executing vertical scanning. Generally, a motor of an ADF (automatic document feeder), etc., used to transport an original has a low capability and the original transport speed is slow. Thus, the vertical scanning speed limit is low in modes 6 to 11 for moving an original as compared with modes 2 to 5 for moving the photoelectric conversion elements, so that changing resolution in the vertical scanning direction interpolation processing when a low magnification with a low resolution is selected.
In such a conventional image reader, the vertical scanning speed is changed and the lines are thinned out or interpolated for decreasing the number of lines in response to the selected resolution and setup magnification, as shown in FIG. 9. However, if the lines are thinned out, the image quality is degraded sharply and if the lines are interpolated, the image of one or more lines previously read must be stored in a memory, for example. The interpolation memory is a factor of increasing image reader costs. To improve the image quality, it is also desirable to deal with change of resolution and magnification by sampling without performing such thinning out or interpolating processing.
On the other hand, as art related to the invention, both a mode of changing the accumulation time of a solid state image sensing device in a television camera for lessening a blur when a mobile image is picked up and a mode of lessening the flicker effect produced by a fluorescent lamp, etc., are realized, for example, in the Unexamined Japanese Patent Application Publication No. Hei 7-36616. If the accumulation time of a solid state image sensing device is changed, the signal level output from the solid state image sensing device changes. In the document, a lens aperture and an analog amplification circuit gain are made variable, whereby the signal level is made constant. However, in such signal processing, when the signal gain is changed, the offset value of the analog amplification circuit also changes, thus the black level of the signal changes and the density changes before and after the accumulation time is changed. In the document, changing of the accumulation time and changing of any other signal level in response to resolution change is not mentioned at all. Further, in the image reader, an original is lighted by an exposure lamp. At this time, change with time such as degradation of the exposure lamp also occurs and the signal gain, offset value, and the like set when the accumulation time is changed need to be found at a predetermined timing rather than set at the shipment time.